In an optical communication system which appends signals to light and transmits light through an optical fiber, a light intensity modulator which modulates the intensity of laser light emitted from a light source to generate an optical signal is used. The light intensity modulator has a configuration in which, for example, a Mach-Zehnder optical waveguide, a modulation electrode, a bias electrode, and the like are formed on an electro-optical crystal substrate, such as lithium niobate (LiNbO3; hereinafter, abbreviated as LN).
The Mach-Zehnder optical waveguide has a waveguide configuration having a splitting portion which splits input light, two arms which allow split light to propagate therethrough, and a multiplexing portion which merges light propagating through the arms. In the multiplexing portion, when two light beams to be merged are in the same phase, an on state in which light waves are output to strengthen each other is placed, and when two light beams are in the opposite phase, an off state in which light waves cancel each other and output light is eliminated is placed. The ratio of the output light intensity in the on state and the output light intensity in the off state is called an extinction ratio and is an important index which represents performance of the light intensity modulator. As the extinction ratio is high, that is, as the difference in output light intensity between the on state and the off state is large, in general, the modulation depth increases, and high-quality optical transmission can be performed.
Most ideally, the output in the off state is zero, and at this time, the extinction ratio is infinite. In order to produce this situation, it is necessary that two light beams to be merged have the same intensity. However, usually, since the split ratio of the splitting portion differs from each other due to a manufacturing error of an optical waveguide or the like, and propagation loss differs between the two arms, the two merged light beams are asymmetrical in intensity. In this case, even if the two light beams are in opposite phases, the two light beams do not completely cancel each other, and the extinction ratio is deteriorated.
As a method which makes two light beams in the multiplexing portion symmetrical in intensity and improves the extinction ratio, for example, a method of irradiating an eximer laser onto an arm having large split power, causing a waveguide defect in order to increase loss, and taking intensity balance with light passing through the other arm, is considered. However, in this method, there is a problem in that loss due to defects depends on wavelength, and the extinction ratio also depends on wavelength.
On the other hand, in a so-called nested modulator (optical SSB modulator) in which sub Mach-Zehnder optical waveguides are respectively provided in two arms of a main Mach-Zehnder optical waveguide, an optical FSK (Frequency Shift Keying) modulator in which RF modulation is performed in each sub Mach-Zehnder optical waveguide to generate optical sideband components (upper and lower sidebands) above and under a frequency, and optical sideband components is switched on the upper side and the lower side and is output as frequency-modulated signal light by selecting a phase corresponding to a data signal in the main Mach-Zehnder optical waveguide has been developed (for example, see PTL 1). In recent years, a light intensity modulator in which a sub Mach-Zehnder optical waveguide is used as a light quantity adjustment unit, and the above-described optical FSK modulator is operated as the above-described light intensity modulator, thereby achieving a high extinction ratio, has been suggested (for example, see NPL 1).